This invention relates generally to the field of telecommunications and is more specifically related to burst modems and improvements therein.
In recent years, the data rates at which communications may be carried out over conventional telephone networks and wiring has greatly increased. These increases are due, in large part, to newly adopted techniques of multiplexing and modulating signals representative of the messages or data being communicated, resulting in greatly improved communication bandwidth. In addition, the carrier frequencies at which such communications are being carried out have also increased in recent years, further improving the bit rate.
In general, the local oscillator frequencies used in a transmitting modem and a receiving modem are not identical. The frequencies of their clocks can be off by as much as 100 ppm. One of the tasks of the receiving modem is to generate a signal that allows it to sample the output of the receiver portion of the modem at the best timing instant. The best timing instant gives the best estimate of the instant to sample the transmitted data. In doing so, the receiver must first acquire and then track the frequency drift of the transmitter""s clock. This invention provides a novel method for the initial acquisition of the phase difference between the transmitted signal and the receiver, and is especially useful in burst modems such as those which are used in CAP/QAM modulation schemes.
Use of quadrature modulation is intended to increase the information-carrying capacity of a modulated signal. One such modulation is Quadrature Amplitude Modulation (QAM), described by Betts et al. in U.S. Pat. No. 5,859,877. QAM involves transmitting data as a sequence of two-dimensional complex signals, i.e. with both in-phase and quadrature components. Each symbol, is assigned a specific pre-defined value according to the data it represents. A set of all of the values available for transmission is termed a constellation, and so resembles a constellation when graphically plotted on a two-dimensional basis.
Another modulation scheme is Carrierless Amplitude Phase modulation (CAP). Receivers using CAP modulation are a bandwidth-efficient means for receiving modulated signals using two-dimensional pass band line code in which the symbol data is organized in I and Q pairs. Knutson et al, U.S. Pat. No. 5930,309, describes a receiver signal processing system for CAP signals. The I and Q data in such a system are filtered with orthogonal I and Q band pass filters having a common pass band. With CAP, processing is done in the pass band of the filters, which eliminates the need for a carrier tracking loop. However, tighter symbol timing constraints is required due to the frequencies of the pulses transmitted. CAP signals can resemble QAM signals except the transmitted data is not spinning or rotating at a carrier frequency.
There are several conventional ways to perform timing recovery in a CAP/QAM system. One method is to implement a phase lock loop (PLL) using a combination of analog and digital techniques as shown in FIG. 1. A Timing Phase Detector (TPD) 101 processes the incoming data samples, s[n], sampled from a signal S(t) by using an analog to digital converter (A/D) 104. One known method, which can perform the function of the TPD 101, is a Band Edge Component Maximization (BECM) process which generates an error signal that is proportional to the difference in phase between the transmitter and receiver clocks. The error signal generated by the TPD 101 is filtered (or averaged) and passed on to a digital to analog (D/A) converter 102. The D/A converter output is used to control the frequency of a voltage controlled oscillator (VCO) 103. The advantage of this approach is that the rest of the receiver does not need to comprehend the timing mismatches. It assumes that the incoming data stream has been sampled at the optimum instant. However, a disadvantage of this approach is the mixing of both analog and digital circuitry.
Another known method for performing timing recovery is an all digital implementation of a PLL as illustrated in FIG. 2. The receiver A/D converter is clocked by a free running oscillator 201. As in the first method, a Timing Phase Detector 101 is used to determine the phase error between the transmitter and receiver clocks according to the frequency of the free running oscillator 201. The error signal from the TPD 101 is then passed to an interpolator 202. The interpolator 202 generates optimally sampled data samples s*[n] based on the signal S(t) sampled by the fee running oscillator at a frequency regulated by the receiver clock. The interpolator 202 adds a fractional delay (less than 1 sample period delay) between the A/D converter and the rest of the receiver processing based on the error signal. The amount of delay is increased or decreased to correct for the transmitter clock drift measured by the TPD. A disadvantage of this method arises when a sample is to be inserted or deleted and the amount of delay required by the interpolator is more than one sample period to be inserted and less than one sample period to be deleted, respectively. As a consequence, when a sample must be inserted, for example the receiver has one sample period less time to process the signal and information may be lost.
Assuming that the equalizer has been converged to reflect the characteristics of the transmission medium, each time the modem is operated, the equalizer must be initialized to account for the phase difference between the signal and the frequency characteristic of the equalizers"" initial state.
Accordingly, a need has arisen in the art for a method and apparatus to provide phase acquisition and initialization for burst modems.
In accordance with the present invention, a method and a device for determining timing phase of a digital signal and for initializing a receiver are provided.
An equalizer is provided for processing a digital signal received by the burst modem receiver. A filter buffer is provided for storing a set of equalizer coefficients to be applied to the equalizer. A buffer manager is provided for storing the coefficients of the equalizer and for performing the acquisition of the timing phase and initialization of the receiver.
The buffer manager is provided for detecting a burst message preamble of the digital, and choosing a desired symbol point from a known burst sequence. The buffer manager samples an output signal from an in-phase equalizer and a quadrature equalizer to form a subset of the preamble, and chooses a selected output pair from the subset.
The buffer manager calculates a desired angle of the desired symbol point and an output angle of the selected output pair, and compares the desired angle with the output angle to obtain an angle difference.
Implementation of a delay for the in-phase equalizer and quadrature equalizer can be provided by the buffer manager based on the angle difference.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.